The method, where a hydrophobic membrane is made hydrophilic by adhering a hydrophilic polymer on a hydrophobic membrane through a process of first impregnating a hydrophobic membrane with a hydrophilic polymer solution, second eliminating this solvent by drying, is well known for a long time, and it may be said that this method is tried for almost all known hydrophilic polymers. Also, it is well known that cellulose derivatives can be used as a hydrophilic polymer. For examples, Examined Japanese Patent (JPB-56-16187(1981)) disclosed uses of methyl cellulose, ethyl cellulose, hydroxy ethyl cellulose and hydroxy propyl cellulose, and Unexamined Japanese Patent (JPA-62-176508(1987)) disclosed a use of hydroxy propyl cellulose of the molecular weight of not less than 10000.
Prior arts have put emphasis on points of making the hydrophobic membranes hydrophilic or making the hydrophilic polymer insoluble, and the arts to prevent leaching of even a quite small amount of the hydrophilic polymer have not sufficiently been studied. However, in fields of medical applications, electronics, etc., since even a quite small quantity of leaching substances from the membranes causes an important problem, it quite often happens that even a quite small quantity of leaching substances from the membranes is avoided essentially, and there are many cases of the prior arts cannot respond sufficiently to these requirements.
The cellulose derivatives used for making the hydrophobic membrane hydrophilic are produced by a chemical modification of a powder-like or flake-like cellulose. Therefore, some cellulose does not completely dissolve and stays in a dispersed state, and, thus, it may result that there is some chemically unmodified cellulose. In the case where the hydrophobic membrane is impregnated with a solution which contains such insufficiently modified cellulose derivatives, it may happen that not only a part of cellulose derivatives which does not dissolve completely clogs pores of the membrane, but the cellulose derivatives may leach out under such a severe condition as a sterilization by autoclaving, since the chemically insufficiently modified cellulose does not adhere molecularly and irreversibly to the surface of the hydrophobic membrane after drying (which means that the molecules once adhered do not detach from the hydrophobic membrane by dissolving again in water).
The subject of this invention is not only to make a hydrophobic membrane hydrophilic but also to provide a hydrophilic membrane from which a leaching substance is reduced as little as possible and which can be used suitably in such fields as medical applications, electronics, etc. where even a quite small quantity of a leaching substance from membranes is prohibited.